Coding apparatus

1. A coding apparatus comprising: a) block forming means for forming a block of sample values of an input signal; b) coding means having a first coding mode in which the block of sample values of the input signal is coded by using data representative of a difference between prediction values and the sample values of the block, and a second coding mode in which the block of the input signal is coded without using the data representative of the difference; c) a buffer memory for temporarily storing the coded data to be read out at a predetermined transmission rate; d) comparing means for comparing a data quantity of the data representative of the difference with a data quantity of data representing the sample values; and e) selecting means for selecting the coding mode according to a level of occupancy of the coded data stored in said buffer memory and a comparison result of said comparing means, said selecting means selecting the second coding mode as a current coding mode after the first coding mode has been selected consecutively for a predetermined number of times according to the level of occupancy of the coded data stored in said buffer memory and the comparison result of said comparing means.

2. An apparatus to claim 1, wherein said selecting means selects the coding mode in units of a predetermined number of blocks.

3. An apparatus according to claim 1, wherein said selecting means selects the first coding mode when said comparing means determines that the data quantity of the data representing the samples values is larger than the data quantity of the data of difference.

4. An apparatus according to claim 1, wherein said coding means performs coding by using a discrete cosine transform (DCT).

5. A coding apparatus comprising: a) coding means for coding an input image signal, said coding means having an intra-picture coding mode and an inter-picture coding mode; b) a buffer memory for temporarily storing the coded image data to be read out at a predetermined transmission rate; and c) selecting means for detecting a level of occupancy of the coded data in said buffer memory and selecting the coding mode in accordance with a detected occupation quantity, said selecting means selecting the intra-picture coding mode as a current coding mode after the inter-picture coding mode has been selected consecutively for a predetermined number of times according to the level of occupancy of the coded data in said buffer memory.

6. An apparatus according to claim 5, further comprising comparing means for comparing the intra-picture data with the inter-picture data, wherein said selecting means selects between the intra-picture coding mode and the inter-picture coding mode in accordance with a comparison result of said comparing means.

7. An apparatus according to claim 5, further comprising block forming means for forming a block of sample values of the input signal, wherein said coding means performs coding in units of the block.

8. An apparatus according to claim 7, further comprising counting means for counting the number of coding operations performed by the inter-picture coding mode, wherein said selecting means selects between the intra-picture coding mode and the inter-picture coding mode in accordance with an output from said counting means.

9. A coding apparatus comprising: a) first block forming means for forming a first block of sample values of an input signal; b) second block forming means for dividing the first block into a plurality of second blocks; c) coding means having a first coding mode in which the second blocks of the input signal are coded by using data representative of a difference between prediction values and values of the input signal of the second blocks, and a second coding mode in which the second blocks of the input signal are coded without using the data representative of the difference; d) a buffer memory for temporarily storing the coded image data to be read out at a predetermined transmission rate; e) detection means for detecting how many times in succession the first coding mode is performed; f) comparing means for comparing a data quantity of the data representative of the difference with a data quantity of data representing the value of the input signal; and g) selecting means for selecting between the first coding mode and the second coding mode in accordance with a level of occupancy of the coded data in said buffer memory, an output of said comparing means and an output of said detection means, said selecting means selecting the second coding mode regardless of the level of occupancy of the coded data in said buffer memory and the output of said comparing means when said detection means detects that the first coding mode has been performed consecutively for a predetermined number of times.

10. An apparatus according to claim 9, wherein said detection means includes counting means for counting the number of coding operations performed on the first block by the first coding mode, wherein said counting means is reset after processing by the second encoding mode is executed.

11. An apparatus according to claim 9, wherein said selecting means selects the coding mode in a unit of the first blocks.

12. An apparatus according to claim 9, wherein each of said first and second coding modes performs coding by using discrete cosine transform (DCT).

13. An apparatus according to claim 12, wherein said second coding means performs coding so that a direct current (DC) component of DCT coefficients of a DCT block is coded according to difference between the DC components of DCT coefficients of the DCT blocks corresponding to the second blocks that constitute the first block.

14. A coding apparatus comprising: a) coding means for coding an input image signal, said coding means having an intra-picture coding mode and an inter-picture coding mode; b) a buffer memory for temporarily storing the coded image data to be read out at a predetermined transmission rate; c) comparing means for comparing intra-picture data with inter-picture data; and d) selecting means for selecting the coding mode in accordance with a level of occupancy of the coded data in said buffer memory and a comparison result of said comparing means, said selecting means selecting the intra-picture coding mode as a current coding mode after the inter-picture coding mode has been selected consecutively for a predetermined number of times according to the level of occupancy of the coded data in said buffer memory and the comparison result of said comparing means.

15. An apparatus according to claim 14, wherein said coding means inter-picture codes a direct current component data of the input image signal transformed by said orthogonal transformation means, in the intra-picture coding mode.

16. A coding method comprising: a) a coding step of coding in input image signal, said coding step having an intra-picture coding mode and an inter-picture coding mode; b) a storing step of temporarily storing the coded image data to be read out at a predetermined transmission rate; and c) a selecting step of detecting a level of occupancy of the coded data stored in said storing step and selecting the intra-picture coding mode in accordance with a detected occupation quantity, said selecting step including a step of selecting the intra-picture coding mode as a current coding mode after the inter-picture coding mode has been selected consecutively for a predetermined number of times.

17. A coding method comprising: a) a coding step of coding an input image signal, said coding step having an intra-picture coding mode and an inter-picture coding mode; b) a storing step of temporarily storing the coded image data to be read out at a predetermined transmission rate; c) a comparing step of comparing intra-picture data with inter-picture data; and d) a selecting step of selecting the coding mode in accordance with a level of occupancy of the coded data stored in said storing step and a comparison result in said comparing step, said selecting step including a step of selecting the intra-picture coding mode as a current coding mode after the inter-picture coding mode has been selected consecutively for a predetermined number of times according to the level of occupancy of the code data in said storing step and the comparison result in said comparing step.

18. A coding method comprising: a) a block forming step of forming a block of sample values of an input signal; b) a coding step having a first coding mode in which the block of sample values of the input signal is coded by using data representative of a difference between prediction values and the sample values of the block, and a second coding mode in which the block of the input signal is coded without using the data representative of the difference; c) a storing step of temporarily storing the coded image data to be read out at a predetermined transmission rate; d) a comparing step of comparing a data quantity of the data representative of the difference with a data quantity of data representing the sample values; and e) a selecting step of selecting a coding mode according to a level of occupancy of the coded data stored in said storing step and a comparison result in said comparing step, said selecting step including a step of selecting the second coding mode as a current coding mode after the first coding mode has been selected consecutively for a predetermined number of times according to the level of occupancy of the coded data stored in said storing step and the comparison result in said comparing step.